Electronic devices capable of presenting images in two formats, that is with part of the image covered in one instance and with less of the image covered in another instance are known. Cellular telephones exist in many such formats, for example, where one half of a clam shell housing has a window for viewing a portion of a display and the full display is revealed when the housing is opened.
Displays for electronic devices can take many forms, including light emitting diodes (LEDs), liquid crystal display (LCD) panels, twisted nematic (TN) and others. New technologies proposed for electronic devices include so called digital paper using electronic ink.
Displays for non-portable electronic devices include cathode ray tubes, plasma displays and projection displays. Projection displays for non-portable electronic devices may be either front projection or rear projection.
When applied to portable electronic devices, any conventional display technology may too large or too energy inefficient for creating thin, conveniently-sized devices for current and next generation applications.